include/babeltrace/object.h

   1 #ifndef BABELTRACE_OBJECT_H
   2 #define BABELTRACE_OBJECT_H
   3
   4 /*
   5  * BabelTrace: common reference counting
   6  *
   7  * Copyright (c) 2015 EfficiOS Inc. and Linux Foundation
   8  * Copyright (c) 2015 Philippe Proulx <pproulx@efficios.com>
   9  * Copyright (c) 2015 Jérémie Galarneau <jeremie.galarneau@efficios.com>
  10  *
  11  * Permission is hereby granted, free of charge, to any person obtaining a copy
  12  * of this software and associated documentation files (the "Software"), to deal
  13  * in the Software without restriction, including without limitation the rights
  14  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  15  * copies of the Software, and to permit persons to whom the Software is
  16  * furnished to do so, subject to the following conditions:
  17  *
  18  * The above copyright notice and this permission notice shall be included in
  19  * all copies or substantial portions of the Software.
  20  *
  21  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  22  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  23  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  24  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  25  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  26  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  27  * SOFTWARE.
  28  */
  29
  30 #ifdef __cplusplus
  31 extern "C" {
  32 #endif
  33
  34 /**
  35 @defgroup refs Reference counting management
  36 @ingroup apiref
  37 @brief Common reference counting management for all Babeltrace objects.
  38
  39 @code
  40 #include <babeltrace/object.h>
  41 @endcode
  42
  43 The macros and functions of this module are everything that is needed
  44 to handle the <strong><em>reference counting</em></strong> of
  45 Babeltrace objects.
  46
  47 Any Babeltrace object can be shared by multiple owners thanks to
  48 <a href="https://en.wikipedia.org/wiki/Reference_counting">reference
  49 counting</a>.
  50
  51 The Babeltrace C API complies with the following key principles:
  52
  53 1. When you call an API function which accepts a Babeltrace object
  54    pointer as a parameter, the API function <strong>borrows the
  55    reference</strong> for the <strong>duration of the function</strong>.
  56
  57    @image html ref-count-user-calls.png
  58
  59    The API function can also get a new reference if the system needs a
  60    more persistent reference, but the ownership is <strong>never
  61    transferred</strong> from the caller to the API function.
  62
  63    In other words, the caller still owns the object after calling any
  64    API function: no function "steals" the user's reference (except
  65    bt_object_put_ref()).
  66
  67 2. An API function which \em returns a Babeltrace object pointer to the
  68    user returns a <strong>new reference</strong>. The caller becomes an
  69    owner of the object.
  70
  71    @image html ref-count-api-returns.png
  72
  73    It is your responsibility to discard the object when you don't
  74    need it anymore with bt_object_put_ref().
  75
  76    For example, see bt_value_array_get().
  77
  78 3. A Babeltrace object pointer received as a parameter in a user
  79    function called back from an API function is a
  80    <strong>borrowed</strong>, or <strong>weak reference</strong>: if you
  81    need a reference which is more persistent than the duration of the
  82    user function, call bt_object_get_ref() on the pointer.
  83
  84    @image html ref-count-callback.png
  85
  86    For example, see bt_value_map_foreach_entry().
  87
  88 The two macros BT_OBJECT_PUT_REF_AND_RESET() and BT_OBJECT_MOVE_REF() operate on \em variables rather
  89 than pointer values. You should use BT_OBJECT_PUT_REF_AND_RESET() instead of bt_object_put_ref() when
  90 possible to avoid "double puts". For the same reason, you should use use
  91 BT_OBJECT_MOVE_REF() instead of performing manual reference moves between
  92 variables.
  93
  94 @file
  95 @brief Reference counting management macros and functions.
  96 @sa refs
  97
  98 @addtogroup refs
  99 @{
 100 */
 101
 102 /**
 103 @brief  Calls bt_object_put_ref() on a variable named \p _var, then
 104         sets \p _var to \c NULL.
 105
 106 Using this macro is considered safer than calling bt_object_put_ref() because it
 107 makes sure that the variable which used to contain a reference to a
 108 Babeltrace object is set to \c NULL so that a future BT_OBJECT_PUT_REF_AND_RESET() or
 109 bt_object_put_ref() call will not cause another, unwanted reference decrementation.
 110
 111 @param[in,out] _var     Name of a variable containing a
 112                         Babeltrace object's address (this address
 113                         can be \c NULL).
 114
 115 @post <strong>If \p _var does not contain \p NULL</strong>,
 116         its reference count is decremented.
 117 @post \p _var contains \c NULL.
 118
 119 @sa BT_OBJECT_MOVE_REF(): Transfers the ownership of a Babeltrace object from a
 120         variable to another.
 121 */
 122 #define BT_OBJECT_PUT_REF_AND_RESET(_var)       \
 123         do {                                    \
 124                 bt_object_put_ref(_var);        \
 125                 (_var) = NULL;                  \
 126         } while (0)
 127
 128 /**
 129 @brief  Transfers the ownership of a Babeltrace object from a variable
 130         named \p _var_src to a variable named \p _var_dst.
 131
 132 This macro implements the following common pattern:
 133
 134   1. Call bt_object_put_ref() on \p _var_dst to make sure the previous reference
 135      held by \p _var_dst is discarded.
 136   2. Assign \p _var_src to \p _var_dst.
 137   3. Set \p _var_src to \c NULL to avoid future, unwanted reference
 138      decrementation of \p _var_src.
 139
 140 @warning
 141 You must \em not use this macro when both \p _var_dst and
 142 \p _var_src contain the same Babeltrace object address and the reference
 143 count of this object is 1. The initial call to bt_object_put_ref() on \p _var_dst
 144 would destroy the object and leave a dangling pointer in \p _var_dst.
 145
 146 @param[in,out] _var_dst Name of the destination variable, containing
 147                         either the address of a Babeltrace object to
 148                         put first, or \c NULL.
 149 @param[in,out] _var_src Name of the source variable, containing
 150                         either the address of a Babeltrace object to
 151                         move, or \c NULL.
 152
 153 @pre <strong>If \p _var_dst and \p _var_src contain the same
 154         value which is not \c NULL</strong>, this object's reference
 155         count is greater than 1.
 156 @post <strong>If \c _var_dst is not \c NULL</strong>, its reference
 157         count is decremented.
 158 @post \p _var_dst is equal to the value of \p _var_src \em before
 159         you called this macro.
 160 @post \p _var_src is \c NULL.
 161
 162 @sa BT_OBJECT_PUT_REF_AND_RESET(): Calls bt_object_put_ref() on a variable, then sets it to \c NULL.
 163 */
 164 #define BT_OBJECT_MOVE_REF(_var_dst, _var_src)  \
 165         do {                                    \
 166                 bt_object_put_ref(_var_dst);    \
 167                 (_var_dst) = (_var_src);        \
 168                 (_var_src) = NULL;              \
 169         } while (0)
 170
 171 /**
 172 @brief  Increments the reference count of the Babeltrace object \p obj.
 173
 174 @param[in] obj  Babeltrace object of which to get a new reference
 175                 (can be \c NULL).
 176 @returns        \p obj
 177
 178 @post <strong>If \c obj is not \c NULL</strong>, its reference
 179         count is incremented.
 180
 181 @sa bt_object_put_ref(): Decrements the reference count of a Babeltrace object.
 182 */
 183 void *bt_object_get_ref(void *obj);
 184
 185 /**
 186 @brief  Decrements the reference count of the Babeltrace object
 187         \p obj.
 188
 189 When the object's reference count reaches 0, the object can no longer
 190 be accessed and is considered \em destroyed.
 191
 192 @remarks
 193 You should use the BT_OBJECT_PUT_REF_AND_RESET() macro instead of calling bt_object_put_ref() since the
 194 former is generally safer.
 195
 196 @param[in] obj  Babeltrace object of which to drop a reference
 197                 (can be \c NULL).
 198
 199 @post <strong>If \c obj is not \c NULL</strong>, its reference
 200         count is decremented.
 201
 202 @sa BT_OBJECT_PUT_REF_AND_RESET(): Calls bt_object_put_ref() on a variable, then sets it to \c NULL.
 203 @sa BT_OBJECT_MOVE_REF(): Transfers the ownership of a Babeltrace object from a
 204         variable to another.
 205 @sa bt_object_get_ref(): Increments the reference count of a Babeltrace object.
 206 */
 207 void bt_object_put_ref(void *obj);
 208
 209 /**
 210 @}
 211 */
 212
 213 #ifdef __cplusplus
 214 }
 215 #endif
 216
 217 #endif /* BABELTRACE_OBJECT_H */